Page 121 - Hydrocarbon Exploration and Production Second Edition

108 Reservoir Fluids

large open pore system. However, since the Karst features are connected downdip
to the waterleg, this is usually followed by rapid and substantial water breakthrough.
A further important reaction is the replacement of the Ca 2+ ion in calcium
carbonate by a magnesium ion. The latter is smaller, hence ‘space’ or porosity is
created in the mineral lattice by the replacement. The resulting mineral is dolomite
and the increase in effective porosity can be as high as 13%. The process can be
expressed as
2CaCO 3 þ Mg 2þ ! CaMgðCO 3 Þ þ Ca 2þ
2
The magnesium ion is made available by migrating pore waters. If the process is
continuous on a geologic time scale, more and more Mg 2+ is introduced to the
system and the porosity reduces again. The rock has been over-dolomitised.
Carbonate reservoirs are usually affected to varying degrees by diagenesis.
However, the process of dissolution and replacement is not limited to carbonates.
Feldspar, for instance, is another family of minerals prone to early alterations.
During drilling and production operations, the chemical equilibrium in the
reservoir pore system may be disturbed. This is particularly true if drilling mud or
injection water enter the formation. The resulting reaction can lead to the preci-
pitation of minerals around the borehole or in the reservoir, and may severely damage
productivity. The compatibility of formation water with fluids introduced during
drilling and production therefore has to be investigated at an early stage.

6.2. Reservoir Fluids

Introduction and Commercial Application: This section introduces the various types
of hydrocarbons which are commonly exploited in oil and gas field developments.
The initial distribution of the fluids in the reservoir must be described to be able to
estimate the hydrocarbons initially in place (HCIIP) in the reservoir. The
relationship between the subsurface volume of HCIIP and the equivalent surface
volume is important in estimating the stock tank oil initially in place (STOIIP) and
the gas initially in place (GIIP). The basic chemistry and physical properties of the
fluid types are used to differentiate the behaviour of the fluids under producing
conditions. For the petroleum and process engineers, a representative description of
the reservoir fluid type is important to predict how the fluid properties will change
with pressure and temperature and is essential for the correct design of the surface
processing facilities. Looking further downstream, the chemical engineer would be
concerned about the composition of the hydrocarbon fluids to determine the yields
of various fractions which may be achieved.

6.2.1. Hydrocarbon chemistry
The fluids contained within petroleum accumulations are mixtures of organic
compounds, which are mostly hydrocarbons (molecules composed of hydrogen and
carbon atoms), but may also include sulphur, nitrogen, oxygen and metal compounds.